Energy storage by means of hydrogen has been gaining increasing importance in recent times. There are now various techniques for storing hydrogen among which a distinction can be made between storage in the gaseous state, the liquid state or in the chemically bound state in the form of metal hydrides. The storage of gaseous or liquid hydrogen often leads to safety problems. Hydrogen storage systems in which hydrogen is stored in the chemically bound state in the form of metal hydrides are therefore advantageous. Such metal hydride hydrogen stores have a storage state and a nonstorage state, between which they can be converted essentially reversibly.
Metal hydride hydrogen stores which are of particular interest are the aluminium hydrides NaAlH4, LiAlH4, Li3ALH6, LiNa2ALH6, Ca(ALH4)2 and borohydrides such as LiBH4, NaBH4, Mg(BH4)2, Ca(BH4)2 and also magnesium hydride (MgH2) due to their relatively high hydrogen storage capacity per unit mass. Although this property and the relatively low price of materials make the abovementioned systems appear to be optimal hydrogen storage systems for transport in the case of hydrogen-driven motor vehicles, their unsatisfactory charging and discharging kinetics have hitherto prevented their use.
Mixing a catalyst into the hydrogen storage material to improve the kinetics in the absorption and desorption of hydrogen is known. WO 2005/019097 A1 discloses the use of a metal carbonate as catalyst for improving the kinetics in the absorption and desorption of hydrogen. WO 2005/068073 A1 discloses the use of a metal-organic compound as catalyst for improving the kinetics in the absorption and desorption of hydrogen. WO 01/53195 A1 discloses the use of metal nitrides and carbides as catalyst for improving the kinetics in the absorption and desorption of hydrogen. WO 00/58206 A1 discloses the use of metal oxides as catalyst for improving the kinetics in the absorption and desorption of hydrogen. DE 10 2005 019 108 B4 discloses a process for producing a hydrogen storage material containing metal hydride, in which metal hydride powder is milled in the presence of from 0.01 to 10% by weight of diamond powder under an inert gas atmosphere, a hydrogen atmosphere or reduced pressure. DE 35 35 378 A1 discloses the use of pulverulent transition metals or transition metal oxides, e.g. palladium, Pd(Al2O3) or palladium-coated powder, for improving the contamination resistance of hydrogen storage material. DE 33 40 492 A1 relates to a process for preparing finely divided, highly reactive magnesium from magnesium hydride, magnesium anthracene and/or derivatives thereof, in which the respective magnesium-containing compound is thermally decomposed in the presence of a reaction partner. WO 01/68515 A1 discloses the use of transition metal compounds in catalytic amounts for improving the hydrogenation/dehydrogenation kinetics of alkali metal alanates. The processes mentioned have the disadvantage of relatively high additional materials costs. In addition, catalysts can no longer be introduced, or can be introduced only with great difficulty, into the hydrogen storage material when this has already been introduced into a tank. The processes mentioned are consequently not suitable at all for regenerating hydrogen storage material.
It is thus an object of the present invention to provide a method of activating or regenerating a hydrogen storage material which contains at least one metal hydride, which process can be carried out inexpensively even when the hydrogen storage material has already been introduced into a tank of the same and does not have the disadvantages of the prior art.
According to the invention, the object is achieved by a method of activating or regenerating a hydrogen storage material which contains at least one metal hydride, in which the at least one metal hydride is brought into contact with an inert solvent and the inert solvent is subsequently removed again. In the context of the invention, an “inert solvent” is one which does not react chemically with the hydrogen storage material under the contact conditions, in particular does not oxidize or reduce said material. The inert solvent is preferably added to the hydrogen storage material in a tank of the same.
The hydrogen storage material preferably contains a complex borohydride, a complex aluminium hydride and/or a metal hydride such as magnesium hydride, even more preferably at least one metal hydride selected from the group consisting of NaAlH4, LiAlH4, Li3AlH6, LiNa2AlH6, Ca(AlH4)2, MgH2 and combinations thereof, most preferably at least one metal hydride selected from the group consisting of LiBH4, NaBH4, Ca(BH4)2, LiAlH4, NaAlH4, Ca(AlH4)2 in combination with MgH2.
The inert solvent is preferably selected from the group consisting of water, an alcohol, more preferably ethanol or tetrahydrofuran, an ether, more preferably diethyl ether, a ketone, more preferably acetone, ammonia, an amine, an amide and combinations thereof. Water and/or ethanol are most preferred.
The period of time between contacting of the hydrogen storage material with the inert solvent and removal of the inert solvent is preferably from 5 seconds to 60 minutes, more preferably from 10 seconds to 60 minutes, most preferably from 10 seconds to 10 minutes.
The solvent is subsequently removed again. This is preferably effected by applying a vacuum or heating the hydrogen storage material which has been contacted with the inert solvent.